Electrical circuit apparatus and methods for assembling same

ABSTRACT

An electrical circuit apparatus ( 300 ) that includes: a substrate ( 330 ) having a ground layer ( 336 ), at least one thermal aperture ( 332 ), and at least one solder aperture ( 334 ); a heat sink ( 310 ); and an adhesive layer ( 320 ) for mechanically coupling the heat sink to the ground layer of the substrate such that at least a portion of the at least one substrate thermal aperture overlaps the heat sink, the adhesive layer having at least one thermal aperture ( 322 ) and at least one solder aperture ( 324 ), wherein aligning the at least one substrate solder aperture with the at least one adhesive layer solder aperture and aligning the at least one substrate thermal aperture with the at least one adhesive layer thermal aperture enables solder wetting in a predetermined area between the heat sink and the ground layer of the substrate.

REFERENCE TO RELATED APPLICATIONS

The present application is related to the following U.S. applicationcommonly owned together with this application by Motorola, Inc.:

Ser. No. 10/677,458, filed Oct. 2, 2003, titled “Electrical CircuitApparatus and Method for Assembling Same” by Waldvogel, et al.

FIELD OF THE INVENTION

The present invention relates generally to methods and electricalcircuit apparatus, wherein components are mounted to a circuit board.

BACKGROUND OF THE INVENTION

When constructing power amplifiers various components must be mounted toa circuit board or substrate. Many of these components are mounted to atop side of the circuit board using a known solder reflow process. Forinstance, a load resistor having at least one input terminal and havinga ground portion or flange may be mounted to the top side of the circuitboard. When mounting a load resistor to a circuit board, three factorsmust be balanced. First, the load resistor must have a proper andsufficient electrical connection to the circuit board, wherein the inputterminals are soldered to the top side of the circuit board and theground flange is sufficiently coupled to a heat sink that is typicallysoldered locally to the underside of the circuit board in an areaprimarily surrounding the load resistor. In addition, a sufficientthermal conduction path must be established between the load resistorand the heat sink. Moreover, load resistors are typically made of aceramic material, which presents a thermal expansion mismatch betweenthe load resistor and the heat sink since the heat sink typically has ahigher coefficient of thermal expansion (CTE) than the ceramic loadresistor. This CTE mismatch can result in local distortion or warping ofthe circuit board after assembly. Solder joint reliability can also besignificantly degraded in a thermal cycling application.

Other components that are mounted to the top side of the circuit boardsuch as, for instance, an inductor coil may require an electricalisolation from a heat sink located below the component. These types ofcomponents may have both input and output terminals that are coupled tothe top side of the circuit board, have heat dissipation needs andrequire a thermal conduction path to the heat sink below, but require anelectrical isolation from the heat sink.

There are a number of methods used for mounting devices such as loadresistors and inductor coils to a circuit board, including a hybridmanufacturing process using fixtures (i.e., a one pass solder reflowprocess) and a two pass solder reflow process. The hybrid manufacturingprocess is typically associated with ceramic circuit boards and possiblywith carrier plates that serve as heat sinks. Due to the fragility ofthe substrate, large fixtures are usually required for its alignment andprotection during processing. The use of fixtures usually forces manualprocessing.

One disadvantage of the hybrid manufacturing process is that it is morecostly than other manufacturing methods primarily due to the added costof the fixtures used in the process and also due to the need for anumber of manual steps that generate a lower production throughput. Anadditional disadvantage is that manufacturing with fixtures produces asignificant variation in part placement and solder attachment due tofixture tolerances or due to fixture degradation with repeated use.

Turning now to the two pass solder reflow process. During the first passof the solder reflow process, a plurality of heat sinks are locallycoupled to the ground layer of a circuit board in areas primarilysurrounding where power components will be mounted. Thereafter, solderis placed in strategic areas on the board, and a plurality ofcomponents, including RF transistors, load resistors and inductor coils,are mounted onto and soldered to the board in a second pass of thereflow solder process.

A primary disadvantage of the two pass reflow process is that itrequires one high-temperature reflow pass with a high meltingtemperature solder alloy, and a second subsequent reflow pass with alower melting temperature solder allow. The first pass exposes thecircuit board to high temperature, which can result in damage such asdistortion. The requirement of two independent passes with differentsolder temperature settings limits manufacturing throughput. The twopass approach also does not lend itself well to no-lead solder becausethe first temperature needed to attach the heat sinks would have toexceed the elevated no-lead solder reflow temperature. This is asignificant disadvantage because no-lead solder attachment may likelybecome a key product differentiator in the near future since somemarkets, especially European markets, are moving toward requiringno-lead solder attachment.

In addition, neither the hybrid manufacturing process nor the two passsolder reflow process addresses the thermal expansion mismatch issuesthat arise when mounting devices such as ceramic load resistors to acircuit board.

Thus, there exists a need for a cost effective method and electricalcircuit apparatus wherein: components may be mounted to a circuit boardwithout the need for fixtures; the process for assembling the electricalcircuit apparatus is compatible with a single pass solder reflow processthat is compatible with, but is not limited to no-lead solder; and anythermal expansion mismatch problems in the electrical circuit apparatusare addressed and, when possible, minimized.

BRIEF DESCRIPTION OF THE FIGURES

A preferred embodiment of the invention is now described, by way ofexample only, with reference to the accompanying figures in which:

FIG. 1 illustrates a topside view of a schematic diagram of a portion ofa substrate in accordance with an embodiment of the present invention;

FIG. 2 illustrates a topside view of a schematic diagram of an adhesivelayer in accordance with an embodiment of the present invention;

FIG. 3 illustrates an exploded view of electrical circuit apparatusincluding a heat sink, an adhesive layer, a substrate, and a ceramicdevice in accordance with an embodiment of the present invention;

FIG. 4 illustrates an assembled topside view of electrical circuitapparatus in accordance with an embodiment of the present invention;

FIG. 5 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus illustrated in FIG. 4 prior to solderwetting;

FIG. 6 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus illustrated in FIG. 4 subsequent to solderwetting;

FIG. 7 illustrates an X-Ray image of an assembled electrical circuitapparatus in accordance with an embodiment of the present inventionafter device population and reflow soldering;

FIG. 8 illustrates a topside view of a schematic diagram of a portion ofa substrate in accordance with an embodiment of the present invention;

FIG. 9 illustrates a topside view of a schematic diagram of an adhesivelayer in accordance with an embodiment of the present invention;

FIG. 10 illustrates an exploded view of electrical circuit apparatusincluding a heat sink, an adhesive layer, a substrate, and a device inaccordance with an embodiment of the present invention;

FIG. 11 illustrates an assembled topside view of electrical circuitapparatus in accordance with an embodiment of the present invention;

FIG. 12 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus illustrated in FIG. 11 prior to solderwetting; and

FIG. 13 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus illustrated in FIG. 11 subsequent to solderwetting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While this invention is susceptible of embodiments in many differentforms, there are shown in the figures and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure is to be considered as an example of the principles of theinvention and not intended to limit the invention to the specificembodiments shown and described. Further, the terms and words usedherein are not to be considered limiting, but rather merely descriptive.It will also be appreciated that for simplicity and clarity ofillustration, elements shown in the figures have not necessarily beendrawn to scale. For example, the dimensions of some of the elements areexaggerated relative to each other. Further, where consideredappropriate, reference numerals have been repeated among the figures toindicate corresponding elements.

The present invention includes a method and electrical circuitapparatus, wherein components may be mounted to a circuit board. In afirst aspect of the invention, a device is mounted to the top side ofthe circuit board and has a primary heat extraction area that may begrounded.

FIG. 1 illustrates a topside view of a schematic diagram of a portion ofa circuit board or substrate 100 in accordance with an embodiment of thepresent invention. In one embodiment, substrate 100 is an organiccircuit board such as a printed circuit board (PCB). However, those ofordinary skill in the art will realize that other substrates (ceramic,for example) may be incorporated. Substrate 100 includes a ground layer(not shown), which may comprise a bottom side of the substrate or may,alternatively, exist internal to the top side and the bottom side of thesubstrate. The ground layer is typically comprised of copper, which maybe coated or plated with a variety of protective layers (e.g., organicsurface coating, tin, nickel or gold).

Substrate 100 may include pads 10 and 20 for enabling a component to bemounted on the topside of substrate 100. For instance, where a loadresistor having at least one input terminal and a ground flange is beingmounted to substrate 100, the input terminals may be coupled to thesubstrate at one pad 10, and the ground flange may be coupled to thesubstrate at the other pad 20.

Substrate 100 further includes at least one but typically a pluralityof, thermal apertures (commonly referred to as thermal vias) 40, thatare electrically and thermally conducting cut-outs extending through thesubstrate, for instance through pad 20, and by which a component may becoupled both electrically and thermally to a heat sink below forgrounding of the component and for heat dissipation of the component. Inone embodiment, a device such as a load resistor may be coupled to theheat sink via thermal apertures 40. However, it is appreciated that thedevice may be any device that is mounted in one region of the substrate,i.e., the top side of the substrate, but that can also be coupled to theheat sink below the substrate such as, for instance, surface mounttransistors or chip capacitors.

Substrate 100 further includes solder apertures 30 that are cut-outsextending through the substrate for accommodating solder addition priorto solder wetting. Solder wetting is defined as the flow of moltensolder due to surface tension forces along a surface or multiplesurfaces away from the initial area of solder addition. The solder maybe in the form of paste, pellets, etc., and may be leaded or no-leadsolder. The placement, size and dimensions of the solder apertures 30are predetermined and assist in causing solder wetting in apredetermined area, for instance, between a heat sink and the groundlayer of the substrate 100. FIG. 1 illustrates two oval shaped solderapertures 30. The placement, size and dimensions of solder apertures 30are exemplary for optimal solder wetting beneath a load resistor.However, those of ordinary skill in the art will realize that dependingupon the particular component being mounted and the desired area forsolder wetting, there may be more or fewer solder apertures in otherlocations on the substrate and having other sizes and dimensions.

FIG. 2 illustrates a topside view of a schematic diagram of an adhesivelayer 200 in accordance with an embodiment of the present invention.Adhesive layer 200 corresponds to substrate portion 100 of FIG. 1 and isused for mechanically coupling at least a portion of one heat sink tothe ground layer of substrate 100 such that at least a portion ofthermal apertures 40 overlap the heat sink.

Adhesive layer 200 is typically comprised of a flexible material withadhesive and cohesive properties that are stable over the hightemperature of the reflow soldering process. The material is typicallyelectrically non-conducting but may also be a conducting material. Inone embodiment, the material is a flexible, pressure sensitive acrylicadhesive. In another embodiment, a flexible liquid or film adhesiverequiring a curing process (e.g., elevated temperature) may be used.Adhesive layer 200 may be manufactured having a predetermined thickness,the purpose of which will be discussed below. Adhesive layer 200includes at least one thermal aperture 240, wherein at least portion ofthe thermal aperture 240 is located beneath pad 20 of substrate 100 andalso beneath at least a portion of a device mounted on top of substrate100. Thermal aperture 240 is likewise a cut-out that extends through theadhesive layer and that has a size and dimensions that enablessufficient coupling between the device and the heat sink but thatprovides electrical isolation where needed between the device and theheat sink. Adhesive layer 200 further includes solder apertures 230 thatcorrespond to solder apertures 30 in substrate 100. Solder apertures 230are cut-outs that likewise extend through adhesive layer 200 foraccommodating solder prior to solder wetting. At least one ventingfeature or aperture 250, which is an additional cut out in the adhesivelayer, may be added in conjunction solder apertures 230. Venting feature250 is typically located on a predetermined area of the adhesive layer200 for enabling solder volatiles to escape for optimal solder wetting.

The placement, size and dimensions of solder apertures 230 arepredetermined and may have portions that are essentially the same sizeand dimensions as that of solder apertures 30 in the substrate so thatthe aligning of solder apertures 30 with solder apertures 230, and thealigning of thermal apertures 40 with thermal aperture 240 provides fora precise cavity for guiding and controlling solder wetting from thesolder apertures (30, 230) to a predetermined area, for instance,between a heat sink and the ground layer of substrate 100.

The venting feature 250 has no corresponding aperture in the substrateand functions to permit volatiles trapped within solder to escape duringsolder wetting. As such, the venting features typically extend to theedge of the heat sink after attachment. In the embodiment illustrated inFIG. 2, there is only one venting feature 250 illustrated, and it islocated adjacent to the adhesive layer thermal aperture 240. However, itis appreciated that additional venting features may be used. Moreover,the size, dimension, number and placement of the venting features may bedetermined, for instance, as a function of the desired solder wettingbetween the substrate ground layer and the heat sink and as a functionof the edge of the heat sink relative to the solder apertures 230 andthe thermal aperture 240. The venting feature in this embodiment is anaperture through the adhesive layer, but it is understood that theventing feature may be one or more holes in the substrate as describedby reference to FIGS. 8 and 10. The adhesive layer may, thus, be die-cutfrom an adhesive film or adhesive coated film for repeatability inproducing the desired thickness and shape of the adhesive layer.

FIG. 3 illustrates an exploded view of electrical circuit apparatus 300in accordance with an embodiment of the present invention. Circuitapparatus 300 includes a heat sink 310, an adhesive layer 320, asubstrate portion 330, and a device 340. Heat sink 310 may be comprisedof a suitable high thermal conductive material such as, for instance,copper or aluminum, that allows wetting of solder and attachment ofadhesive materials selected for the circuit apparatus assembly process.However, in another embodiment of the present invention, heat sink 310may be comprised of a material having a coefficient of thermal expansion(“CTE”) that is essentially matched to that of device 340 to minimizethermal expansion differences between device 340 and the heat sink 310.Heat sink 310 has two primary sides 312 and 314. At side 312: substrateportion 330 is attached using adhesive layer 320; device 340 is coupledusing solder; and heat is input into heat sink 310 for dissipation. Theopposite side 314 is the primary region of heat extraction from circuitapparatus 300, as well as the primary mounting surface for circuitapparatus 300. In one embodiment, the size of the heat sink is largerthan that of the heat dissipating device (e.g., device 340), such thatdesirable heat spreading can be achieved.

Adhesive layer 320 is in accordance with the adhesive layer as describedby reference to FIG. 2. Accordingly, adhesive layer 320 includes athermal aperture 322, solder apertures 324, and a venting feature 350.Substrate portion 330 is in accordance with the substrate portion asdescribed by reference to FIG. 1. Accordingly, substrate portion 330includes thermal apertures 332, solder apertures 334, and a ground layer336. Substrate 330 also typically includes a plurality of pads 338 onthe topside of the substrate onto which the device 340 may be coupledand through which the substrate thermal apertures may extend. In theembodiment illustrated in FIG. 3, ground layer 336 comprises the bottomside of substrate 330. However, it is realized that ground layer 336 maybe internal to substrate 330, wherein substrate 330 would furtherinclude a recess for exposing the ground layer, the recess typicallyhaving dimensions that are slightly larger than that of heat sink 310.

Finally, device 340 may comprise at least one input terminal 342 and aground portion or flange 346. In one embodiment, device 340 is a loadresistor. However, it is appreciated that device 340 may also be anydevice that is mounted on the top side of the substrate portion 330 butthat may also be coupled to the heat sink 310. It is also appreciatedthat the load resistor is typically a ceramic device consisting ofmaterials such as aluminum oxide, beryllium oxide or aluminum nitridehaving a low CTE, typically in the range 4 to 9 ppm/° C. Accordingly, inanother aspect of the present invention heat sink 310 maybe selectedhaving a material with a CTE that essentially matches that of theceramic load resistor to minimize thermal expansion mismatch between thecomponent and the heat sink.

The above-described elements of circuit apparatus 300 may be assembledas follows in accordance with the present invention. Adhesive layer 320is aligned with substrate 330 such that adhesive layer thermal aperture322 is aligned with substrate thermal apertures 332 and adhesive layersolder apertures 324 are aligned with substrate solder apertures 334.Heat sink 310 is mechanically coupled to the ground layer 336 ofsubstrate 330 using adhesive layer 320, such that heat sink 310 isaligned with substrate 330 and at least a portion of thermal apertures322 and 332 overlap heat sink 310. In the embodiment illustrated in FIG.3, heat sink 310 is coupled locally to substrate 330 in an area thatcompletely surrounds device 340 for providing an optimal thermalconduction path.

Solder is placed on the substrate pads (and thereby on at least aportion of the substrate thermal apertures), and into at least a portionof the adhesive layer solder apertures 324 for subsequent solder wettingto couple the device input terminals 342 to the substrate pads and tocouple the device flange 346 to the heat sink 310, thereby grounding thedevice 340. Typically, solder paste is screen-printed on the substratepads and into the solder apertures 324. However, in other embodiments,other forms of solder, e.g., solder pellets or pre-forms, may beimplemented. It is further appreciated that during solder addition,solder may also be added to at least a portion of the substrate solderapertures 334. In fact, typically both the substrate and adhesive layersolder apertures (324, 334) are filled during solder addition.

The device 340 is mounted onto the topside of substrate 330 such that atleast one input terminal 342 comes into contact with the solder on thecorresponding pad on the topside of substrate 330 and at least a portionof the device flange 346 covers at least a portion of substrate thermalapertures 332. Population of the substrate 330 with the device 340 maybe done manually, but is typically done using an automated process forefficiency and cost effectiveness during the manufacturing process. Thepopulated substrate 330 may be placed in a reflow oven and thereaftercooled, wherein: a solder connection between the device inputs terminal342 and the corresponding substrate pad is completed; solder wetsthrough a least a portion of the substrate thermal apertures 332; andsolder wets from the solder apertures (324, 334) into the cavity betweenthe ground layer 336 and the heat sink 310 to complete the grounding andthermal coupling of device 340.

In one embodiment, at least a portion of the steps of the methodaccording to the present invention described above may be performed aspart of an automated process, and ideally all of the steps are soperformed. However, it is realized that any of the above described stepsin various combinations may be performed manually or as part of anautomated process.

Mechanical attachment of the heat sink to the substrate prior to refloweliminates the need for fixtures to hold the heat sink in place duringthe surface mount technology (SMT) processing and adds robustness duringthe assembly process for handling of the circuit apparatus assembly.Assembly of the electrical circuit apparatus may be performed during asingle pass reflow process for the thermal coupling and device groundingand topside SMT attachment, thereby lending itself well with the use ofno-lead solder or leaded solder.

The adhesive layer solder apertures, the substrate ground layer and thewettable heat sink surface promote wetting of solder from the solderapertures to areas of critical thermal transfer and RF grounding duringreflow. High surface energy surfaces above (substrate ground layer) andbelow (heat sink) promote the wetting of solder to the open spacebetween the two wettable surfaces. These surfaces also provide idealadhesive bonding surfaces yielding high adhesion strength between theheat sink and the substrate. During solder addition, solder fills manyof the substrate thermal apertures for subsequent solder wetting toproduce a good thermal conduction path from the device to the heat sink,as well as a sufficient ground connection.

Use of a film adhesive with controlled thickness produces a highlyrepeatable separation, resulting in lower variation of this criticaldimension for the manufacturing process. A venting feature may becreated by extending the adhesive cut-out to the edge of the circuitboard or through at least one venting hole formed in the circuit board.This venting feature further promotes optimal solder filling in theseparation by allowing solder paste volatiles to escape. The size andshape of the solder apertures for the paste also defines the volume ofmolten solder to fill the separation and is easily controlled tooptimize thermal coupling and RF grounding. The combination of thiscontrol of solder volume and the termination of the region of two highsurface energy surfaces created by the cut-outs in the adhesiverestricts the flow of molten solder to the region of interest. Theresulting ground layer-to-heat sink solder connection producesrepeatable thermal and RF ground paths from the load resistor to theheat sink, wherein the ground paths are directly beneath the body of thedevice for optimal electrical performance. A high thermal conductivity,low CTE heat sink is used in the electrical circuit apparatus to managethe power dissipation needs of the ceramic load resistor while alsominimizing thermal expansion differences with the low CTE ceramic loadresistor.

Since the bulk of the attachment of the heat sink to the substrate isaccomplished using a low-stiffness adhesive, thermal expansiondifferences between the heat sink and matching components on top of thesubstrate (e.g., ceramic components, such as RF matching capacitors,that have a much lower coefficient of thermal expansion than the heatsink) are decoupled, thus improving the reliability of the componentsand corresponding solder joints. Moreover, the thermal apertures enablea good thermal conduction path between the ground flange of the deviceand the heat sink.

FIG. 3, for simplicity, illustrates a portion of a substrate having onecomponent mounted thereon using methods described above in accordancewith the present invention. However, those of ordinary skill in the artwill realize that a substrate typically has a plurality of components.Those of ordinary skill in the art will further realize that althoughFIG. 3 illustrates heat sink 310 being coupled locally to substrate 330beneath only one device 340, typically heat sink 310 is coupled locallybeneath a plurality of devices for efficiency in manufacturing and tominimize manufacturing costs. In addition, FIG. 3 only shows one heatsink being coupled to substrate 330. However, it is appreciated that aplurality of heat sinks may be coupled to the substrate.

FIG. 4 illustrates an assembled topside view of an electrical circuitapparatus 400, in accordance with the electrical circuit apparatusillustrated in FIG. 3, subsequent to a solder paste screening and devicepopulation but prior to solder wetting. Illustrated in FIG. 4 is thetopside of a device 410 that has been mounted onto a substrate 414,wherein the at least one device input terminal 420 has made contact withthe solder on a pad (not shown) on the top side of the substrate 414,and a ground flange (not shown) has made contact with solder on anothersubstrate pad 430 on the topside of the substrate 414. Also illustratedare two solder apertures 440 that have been filled with solder usingknown methods, and a cross-section line labeled A—A illustrates across-sectional area of the electrical circuit apparatus 400 that willbe discussed in detail by reference to FIGS. 5 and 6.

FIG. 5 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus 400 illustrated in FIG. 4 prior to solderwetting. This cross-sectional view illustrates a device 510 having atleast one input terminal 514 and a ground flange 532 coupled to pads 522on a substrate 524 via a solder layer 520. At least a portion of groundflange 532 is mounted over a plurality of thermal apertures 564 thatextend through substrate 524. A ground layer 528 of substrate 524 ismechanically coupled to a heat sink 550 via an adhesive layer 540,wherein the adhesive layer 540 creates a precise cavity 544 between theground layer 528 and heat sink 550. Further illustrated is solder 560that has been added using known methods into a solder aperture 570.

FIG. 6 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus 400 illustrated in FIG. 4 subsequent tosolder wetting. Those elements that are identical to the elementsillustrated in FIG. 5 are correspondingly identically labeled in FIG. 6and for the sake of brevity are not described again here. FIG. 6,however, further illustrates solder wetting 610 of solder 520 into atleast a portion of the thermal apertures and of solder from solderaperture 570 into an area between the ground layer 528 of the substrate524 and the heat sink 550, for thermal coupling and grounding betweenthe device flange 532 and the heat sink 550, directly beneath the bodyof device 510.

FIG. 7 illustrates an X-Ray image of an assembled electrical circuitapparatus in accordance with the present invention after devicepopulation and reflow soldering. This X-Ray image clearly shows howsolder has wetted from solder apertures 710 and within thermal apertures720 to produce ideal solder connections between the ground layer of thesubstrate and the heat sink and between the ground flange of the deviceand the heat sink in an area directly beneath the body of the device.

A number of exemplary advantages over the prior art can be realizedusing the method and electrical circuit apparatus of the presentinvention, wherein power devices may be mounted to a circuit board.These advantages include, but are not limited to: (1) minimization ofthermal mismatch problems by matching the CTE of, for instance, a loadresistor and the heat sink; (2) a good thermal path from the bottom ofthe device, though a plurality of thermal apertures, to a heat sinkdirectly below; (3) repeatable solder attachment of the ground layer ofthe circuit board to the heat sink, directly under the device; (4)mechanical attachment of the heat sink to the circuit board to addrobustness to assembly for handling and subsequent module assembly; (5)elimination of the need for fixtures in a one-step or single pass reflowsoldering process that lends itself to no-lead solder or leaded solder;and (6) solder attachment for thermal management and for RF groundingcan be accomplished during SMT attachment of other components to thecircuit board without requiring additional process steps.

In another aspect of the present invention, a device is mounted to thetop side of a circuit board and requires heat dissipation via a heatsink mounted below the circuit board but also requires electricalisolation from the heat sink. FIG. 8 illustrates a topside view of aschematic diagram of a portion of a circuit board or substrate 800 inaccordance with an embodiment of the present invention. In oneembodiment, substrate 800 is an organic circuit board such as a printedcircuit board (PCB). However, those of ordinary skill in the art willrealize that other substrates (ceramic, for example) may beincorporated. Substrate 800 includes a ground layer (not shown), whichmay comprise a bottom side of the substrate or may, alternatively, existinternal to the top side and the bottom side of the substrate. Theground layer is typically comprised of copper, which may be coated orplated with a variety of protective layers (e.g., organic surfacecoating, tin, nickel or gold).

Substrate 800 may also include pads 820 for enabling a component to bemounted on the topside of substrate 800. For instance, where an inductorcoil having at least one input terminal and at least one output terminalis being mounted to substrate 800, the input terminals may be coupled tothe substrate at one pad 820, and the output terminals may be coupled tothe substrate at the other pad 820.

Substrate 800 further includes solder apertures 830 that are cut-outsextending through the substrate for accommodating solder addition priorto solder wetting. The placement, size and dimensions of the solderapertures 830 are predetermined and assist in causing solder wetting ina predetermined area, for instance, between a heat sink and the groundlayer of the substrate 800. FIG. 8 illustrates two oval shaped solderapertures 830. The placement, size and dimensions of solder apertures830 are exemplary for optimal solder wetting beneath an inductive coil.However, those of ordinary skill in the art will realize that dependingupon the particular component being mounted and the desired area forsolder wetting, there may be more or fewer solder apertures in otherlocations on the substrate and having other sizes and dimensions.Substrate 800 may also include one or more venting holes 840 extendingthrough the substrate and located in predetermined areas on thesubstrate. The venting holes 840 promote optimal solder wetting betweenthe ground layer of the substrate and a heat sink by allowing soldervolatiles to escape during solder reflow. FIG. 8 illustrates twocircular shaped venting holes. However, it is appreciated that there maybe more or fewer venting holes having different sizes, shapes,dimensions and locations on the substrate depending on the desired areaof solder wetting. Moreover, it is appreciated that the venting ofsolder volatiles may, likewise, be accomplished using one or moreventing apertures in the adhesive layer as described above by referenceto FIGS. 2 and 3.

FIG. 9 illustrates a topside view of a schematic diagram of an adhesivelayer 900 in accordance with an embodiment of the present invention.Adhesive layer 900 corresponds to substrate portion 800 of FIG. 8 and isused for mechanically coupling at least a portion of one heat sink tothe ground layer of substrate 800.

Adhesive layer 900 is typically comprised of a flexible material withadhesive and cohesive properties that are stable over the hightemperature of the reflow soldering process. The material is typicallyelectrically non-conducting but may also be a conducting material. Inone embodiment, the material is a flexible, pressure sensitive acrylicadhesive. In another embodiment, a flexible liquid or film adhesiverequiring a curing process (e.g., elevated temperature) may be used.Adhesive layer 900 may be manufactured having a predetermined thickness.Adhesive layer 900 includes solder apertures 930 that correspond tosolder apertures 830 and venting holes 840 in substrate 800. Solderapertures 930 are cut-outs that likewise extend through adhesive layer900 for accommodating solder prior to solder wetting.

The placement, size and dimensions of solder apertures 930 arepredetermined so that the aligning of solder apertures 830 and ventingholes 840 with solder apertures 930 provides for a precise cavity forguiding and controlling solder wetting from the solder apertures (830,930) to a predetermined area, for instance, between a heat sink and theground layer of substrate 800. The adhesive layer may, thus, be die-cutfrom an adhesive film or adhesive coated film for repeatability inproducing the desired thickness and shape of the adhesive layer.

FIG. 10 illustrates an exploded view of electrical circuit apparatus1000 in accordance with an embodiment of the present invention. Circuitapparatus 1000 includes a heat sink 1010, an adhesive layer 1020, asubstrate portion 1030, and a device 1040. Heat sink 1010 is comprisedof a suitable high thermal conductive material such as, for instance,copper or aluminum, that allows wetting of solder and attachment ofadhesive materials selected for the circuit apparatus assembly process.Heat sink 1010 has two primary sides 1012 and 1014. At side 1012:substrate portion 1030 is attached using adhesive layer 1020; device1040 is attached using solder; and heat is input into heat sink 1010 fordissipation. The opposite side 1014 is the primary region of heatextraction from circuit apparatus 1000, as well as the primary mountingsurface for circuit apparatus 1000. In one embodiment, the size of theheat sink is larger than that of the heat dissipating device (e.g.,device 1040), such that desirable heat spreading can be achieved.

Adhesive layer 1020 is in accordance with the adhesive layer asdescribed by reference to FIG. 9. Accordingly, adhesive layer 1020includes solder apertures 1024. Substrate portion 1030 is in accordancewith the substrate portion as described by reference to FIG. 8.Accordingly, substrate portion 1030 includes venting holes 1032, solderapertures 1034, and a ground layer 1036. Substrate 1030 also typicallyincludes a plurality of pads 1036 on the topside of the substrate ontowhich the device 1040 may be coupled. In the embodiment illustrated inFIG. 10, ground layer 1036 comprises the bottom side of substrate 1030.However, it is realized that ground layer 1036 may be internal tosubstrate 1030, wherein substrate 1030 would further include a recessfor exposing the ground layer, the recess typically having dimensionsthat are slightly larger than that of heat sink 1010. Finally, device1040 may comprise at least one input terminal 1042 and at least oneoutput terminal 1044. Device 1040 may be, for instance, an inductivecoil or any device that is mounted to the top side substrate portion1030 and that requires thermal coupling with heat sink 1010 but thatfurther requires an electrical isolation from heat sink 1010.

The above-described elements of circuit apparatus 1000 may be assembledas follows in accordance with the present invention. Adhesive layer 1020is aligned with substrate 1030 such that solder apertures 1024 arealigned with solder apertures 1034 and with venting holes 1032. Heatsink 1010 is mechanically coupled to the ground layer 1036 of substrate1030 using adhesive layer 1020, such that it is aligned with substrate1030. In the embodiment illustrated in FIG. 10, heat sink 1010 iscoupled locally to substrate 1030 in an area that completely surroundsdevice 1040 for providing an optimal thermal conduction path.

Solder is placed on the substrate pads and into at least a portion ofthe adhesive layer solder apertures 1024 for subsequent solder wettingto couple the device input and output terminals (1042, 1044) to thepads, to couple the ground layer 1036 to the heat sink 1010 and toproduce thermal coupling of device 1040. Typically, solder paste isscreen-printed on the substrate pads and into the adhesive layer solderapertures 1024. However, in other embodiments, other forms of solder,e.g., solder pellets or pre-forms, may be implemented. It is furtherappreciated that during solder addition, solder may also be added to atleast a portion of the substrate solder apertures 1034. In fact,typically both the substrate and adhesive layer solder apertures (1024,1034) are filled during solder addition.

Device 1040 is mounted onto the topside of substrate 1030 such that atleast one input terminal 1042 comes into contact with the solder on thecorresponding pads on the topside of substrate 1030 and at least oneoutput terminal 1044 comes into contact with the solder on thecorresponding pads on the topside of substrate 1030. Population of thesubstrate 1030 with the device 1040 may be done manually, but istypically done using an automated process for efficiency and costeffectiveness during the manufacturing process.

The populated substrate 1030 may be placed in a reflow oven andthereafter cooled, wherein a solder connection between the deviceterminals (1042, 1044) and the pads is completed and solder wets fromthe solder apertures (1024, 1034) into the cavity between the groundlayer 1036 and the heat sink 1010 to produce a thermal path betweendevice 1040 and heat sink 1010.

In one embodiment, at least a portion of the steps of the methodaccording to the present invention described above may be performed aspart of an automated process, and ideally all of the steps are soperformed. However, it is realized that any of the above described stepsin various combinations may be performed manually or as part of anautomated process.

Mechanical attachment of the heat sink to the substrate prior to refloweliminates the need for fixtures to hold the heat sink in place duringthe SMT processing and adds robustness during the assembly process forhandling of the circuit apparatus assembly. Assembly of the electricalcircuit apparatus may be performed during a single pass reflow processfor the thermal coupling and topside SMT attachment, thereby lendingitself well with the use of no-lead solder or leaded solder.

The adhesive layer solder apertures, the substrate ground layer and thewettable heat sink surface promote wetting of solder from the solderapertures to areas of critical thermal coupling during reflow. Highsurface energy surfaces above (substrate ground layer) and below (heatsink) promote the wetting of solder to the open space between the twowettable surfaces. These surfaces also provide ideal adhesive bondingsurfaces yielding high adhesion strength between the heat sink and thesubstrate.

Use of a film adhesive with controlled thickness produces a highlyrepeatable separation, resulting in lower variation of this criticaldimension for the manufacturing process. A venting feature may becreated by extending the adhesive cut-out region to the edge of thecircuit board or through at least one venting hole formed in the circuitboard. This feature further promotes optimal solder filling in theseparation by allowing solder paste volatiles to escape during reflow.The size and shape of the solder apertures for the paste also definesthe volume of molten solder to fill the separation and is easilycontrolled to optimize heat transfer. The combination of this control ofsolder volume and the termination of the region of two high surfaceenergy surfaces created by the cut-outs in the adhesive restricts theflow of molten solder to the region of interest. The resulting groundlayer-to-heat sink solder connection produces a repeatable thermal pathfrom the heat dissipation component to the heat sink.

Since the bulk of the attachment of the heat sink to the substrate isaccomplished using a low-stiffness adhesive, thermal expansiondifferences between the heat sink and matching components on top of thesubstrate (e.g., ceramic components, such as RF matching capacitors,that have a much lower coefficient of thermal expansion than the heatsink) are decoupled, thus improving the reliability of the componentsand corresponding solder joints. Moreover, the thermal apertures enablea good thermal conduction path between the bottom of the device and theheat sink.

FIG. 10, for simplicity, illustrates a portion of a substrate having onecomponent mounted thereon using methods described above in accordancewith the present invention. However, those of ordinary skill in the artwill realize that a substrate typically has a plurality of componentsmounted thereon and includes, for instance, power components, ceramicload resistors, inductive coils and other components. Thus, in anotheraspect of the present invention, an electrical circuit apparatus mayhave heat sinks incorporated therein having different CTE values. Forinstance at least one heat sink having a low CTE may be coupled to thesubstrate beneath a low-CTE component such as, for instance, a ceramicload resistor, and at least one heat sink having a higher CTE may becoupled to the substrate under another high-CTE component such as, forinstance, a radio frequency (“RF”) or power transistor. Those ofordinary skill in the art will further realize that although FIG. 10illustrates heat sink 1010 being coupled locally to substrate 1030beneath only one device 1040, typically heat sink 1010 is coupledlocally beneath a plurality of devices for efficiency in manufacturingand to minimize manufacturing costs. In addition, FIG. 10 only shows oneheat sink being coupled to substrate 1030. However, it is appreciatedthat a plurality of heat sinks may be coupled to the substrate.

FIG. 11 illustrates an assembled topside view of an electrical circuitapparatus 1100, in accordance with the electrical circuit apparatusillustrated in FIG. 10, subsequent to a solder paste screening anddevice population but prior to solder wetting. Illustrated in FIG. 11 isthe topside of a device 1110 that has been mounted onto a substrate1114, wherein the topside of at least one device input terminal 1120 andat least one device output terminal 1130 have made contact with thesolder on the input and output pads (not shown) on the topside of thesubstrate 1114. Also illustrated are two venting holes 1150, two solderapertures 1140 that have been filled with solder using known methods,and a cross-section line labeled A—A that illustrates a cross-sectionalarea of the electrical circuit apparatus 1100 that will be discussed indetail by reference to FIGS. 12 and 13.

FIG. 12 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus 1100 illustrated in FIG. 11 prior to solderwetting. This cross-sectional view illustrates at least one outputterminal 1216 of a device coupled to a pad 1222 on a substrate 1224 viaa solder layer 1220. A ground layer 1228 of substrate 1224 ismechanically coupled to a heat sink 1250 via an adhesive layer 1240,wherein the adhesive layer 1240 creates a precise cavity 1244 betweenthe ground layer 1228 and heat sink 1250. Further illustrated is aventing hole 1280 and solder 1260 that has been added, using knownmethods, into a solder aperture 1270.

FIG. 13 illustrates a cross-sectional view at a section A—A of theelectrical circuit apparatus 1100 illustrated in FIG. 11 subsequent tosolder wetting. Those elements that are identical to the elementsillustrated in FIG. 12 are correspondingly identically labeled in FIG.13 and for the sake of brevity are not described again here. FIG. 13,however, further illustrates solder wetting 1310 from solder aperture1270 toward venting hole 1280 in an area between the ground layer 1228of the substrate 1224 and the heat sink 1250, for thermal coupling ofthe device to heat sink 1250.

A number of exemplary advantages over the prior art can be realizedusing the method and electrical circuit apparatus of the presentinvention, wherein power devices may be mounted to a circuit board.These advantages include, but are not limited to: (1) minimization ofCTE mismatch problems, for instance, between ceramic capacitors and theheat sink, by using a flexible, low-stiffness adhesive; (2) repeatablesolder attachment of the ground layer of the circuit board to the heatsink directly under the device; (3) electrical isolation of deviceterminals from ground, i.e., the heat sink; (4) mechanical attachment ofthe heat sink to the circuit board to add robustness to assembly forhandling and subsequent module assembly; (5) elimination of the need forfixtures in a one-step or single pass reflow soldering process thatlends itself to no-lead solder or leaded solder; and (6) solderattachment for thermal management can be accomplished during SMTattachment of other components to the circuit board without requiringadditional process steps.

While the invention has been described in conjunction with specificembodiments thereof, additional advantages and modifications willreadily occur to those skilled in the art. The invention, in its broaderaspects, is therefore not limited to the specific details,representative apparatus, and illustrative examples shown and described.Various alterations, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. Thus, itshould be understood that the invention is not limited by the foregoingdescription, but embraces all such alterations, modifications andvariations in accordance with the spirit and scope of the appendedclaims.

1. An electrical circuit apparatus comprising; a substrate comprising atop side, a ground layer, at least one thermal aperture, and at leastone solder aperture; a heat sink; and an adhesive layer for mechanicallycoupling said heat sink to the ground layer of said substrate such thatat least a portion of said at least one substrate thermal apertureoverlaps said heat sink, said adhesive layer comprising at least onethermal aperture and at least one solder aperture, wherein aligning theat least one substrate solder aperture with the at least one adhesivelayer solder aperture and aligning the at least one substrate thermalaperture with the at least one adhesive layer thermal aperture enablessolder wetting in a predetermined area between said heat sink and theground layer of said substrate.
 2. The apparatus of claim 1 furthercomprising a device coupled to the top side of said substrate such thatat least a portion of said device covers at least a portion of said atleast one substrate thermal aperture, and wherein said device is coupledto said heat sink via at least a portion of said at least one substratethermal aperture.
 3. The apparatus of claim 2, wherein said devicecomprises at least one input terminal and a ground flange, and at leasta portion of said predetermined area is located beneath the groundflange of said device.
 4. The apparatus of claim 2, wherein said heatsink has a coefficient of thermal expansion that is essentially matchedto that of said device.
 5. The apparatus of claim 2, wherein said deviceis a ceramic device.
 6. The apparatus of claim 2, wherein said device iscoupled to said substrate and said heat sink and said solder wettingoccurs during a single pass solder reflow process.
 7. The apparatus ofclaim 6, wherein said solder reflow process uses a no-lead solder. 8.The apparatus of claim 6, wherein said solder reflow process uses aleaded solder.
 9. The apparatus of claim 1, wherein said adhesive layercomprises an electrically non-conducting material.
 10. The apparatus ofclaim 9, wherein said electrically non-conductive material is acrylic.11. The apparatus of claim 1, wherein said substrate is an organiccircuit board.
 12. The apparatus of claim 1, wherein said adhesive layerhas a predetermined thickness.
 13. The apparatus of claim 1, whereinsaid adhesive layer further comprises at least one venting aperturelocated on a predetermined area of said adhesive layer to enable theescape of solder volatiles during solder reflow.
 14. The apparatus ofclaim 13, wherein said predetermined area is adjacent to said adhesivelayer thermal aperture.
 15. An electrical circuit apparatus comprising;a substrate comprising a top side, a ground layer, at least one thermalaperture, and at least one solder aperture; a heat sink; an adhesivelayer for mechanically coupling said heat sink to the ground layer ofsaid substrate such that at least a portion of said at least onesubstrate thermal aperture overlaps said heat sink, said adhesive layercomprising at least one thermal aperture and at least one solderaperture; and a device coupled to the top side of said substrate suchthat at least a portion of said device covers at least a portion of saidat least one substrate thermal aperture, and wherein said device iscoupled to said heat sink via at least a portion of said at least onethermal aperture, and wherein aligning the at least one substrate solderaperture with the at least one adhesive layer solder aperture andaligning the at least one substrate thermal aperture with the at leastone adhesive layer solder aperture enables solder wetting in apredetermined area between said heat sink and the ground layer of saidsubstrate, and wherein at least a portion of said predetermined area islocated beneath said device.
 16. An electrical circuit apparatuscomprising; an organic circuit board comprising a top side, a groundlayer, at least one thermal aperture, and at least one solder aperture;a heat sink; an adhesive layer for mechanically coupling said heat sinkto the ground layer of said substrate such that at least a portion ofsaid at least one substrate thermal aperture overlaps said heat sink,said adhesive layer having a predetermined thickness and comprising atleast one thermal aperture, at least one solder aperture and at leastone venting aperture adjacent to the at least one adhesive layer thermalaperture; and a device comprising at least one input terminal and aground flange and coupled to the top side of said substrate such that atleast a portion of said device covers at least a portion of said atleast one substrate thermal aperture, and wherein said device is coupledto said heat sink via at least a portion of said at least one substratethermal aperture, and wherein aligning the at least one substrate solderaperture with the at least one adhesive layer solder aperture andaligning the at least one substrate thermal aperture with the at leastone adhesive layer thermal aperture enables solder wetting in apredetermined area between said heat sink and the ground layer of saidsubstrate, and wherein at least a portion of said predetermined area islocated beneath the ground flange of said device.
 17. An electricalcircuit apparatus comprising; a substrate comprising a top side, aground layer, and at least one solder aperture; a heat sink; and anadhesive layer for mechanically coupling said heat sink to the groundlayer of said substrate, said adhesive layer comprising at least onesolder aperture, wherein aligning the at least one substrate solderaperture with the at least one adhesive layer solder aperture enablessolder wetting in a predetermined area between said heat sink and theground layer of said substrate.
 18. The apparatus of claim 17 furthercomprising a device coupled to the topside of said heat sink.
 19. Theapparatus of claim 18, wherein at least a portion of said predeterminedarea is located beneath said device.
 20. The apparatus of claim 18,wherein said device is an inductive coil.
 21. The apparatus of claim 17,wherein said adhesive layer comprises an electrically non-conductingmaterial.
 22. The apparatus of claim 21, wherein said electricallynon-conductive material is acrylic.
 23. The apparatus of claim 17,wherein said substrate is an organic circuit board.
 24. The apparatus ofclaim 17, wherein said adhesive layer has a predetermined thickness. 25.The apparatus of claim 17, wherein said substrate further comprises atleast one venting hole that is aligned with said at least one adhesivelayer solder aperture to enable the escape of solder volatiles duringsaid solder wetting.
 26. An electrical circuit apparatus comprising; asubstrate comprising a top side, a ground layer, and at least one solderaperture; a heat sink; an adhesive layer for mechanically coupling saidheat sink to the ground layer of said substrate, said adhesive layercomprising at least one solder aperture; and a device coupled to thetopside of said heat sink, wherein aligning the at least one substratesolder aperture with the at least one adhesive layer solder apertureenables solder wetting in a predetermined area between said heat sinkand the ground layer of said substrate, and wherein at least a portionof said predetermined area is located beneath said device.
 27. Anelectrical circuit apparatus comprising; an organic circuit comprising atop side, a ground layer, at least one solder aperture, and at least oneventing hole; a heat sink; an adhesive layer for mechanically couplingsaid heat sink to the ground layer of said substrate, said adhesivelayer having a predetermined thickness and comprising at least onesolder aperture; and a device coupled to the topside of said heat sink,wherein aligning the at least one substrate solder aperture and the atleast one venting hole with the at least one adhesive layer solderaperture enables solder wetting in a predetermined area between saidheat sink and the ground layer of said substrate, and wherein at least aportion of said predetermined area is located beneath said device. 28.An electrical circuit apparatus comprising; a substrate comprising a topside, a ground layer, and at least one solder aperture; a first heatsink having a first coefficient of thermal expansion (“CTE”); a secondheat sink having a second CTE that is lower than said first CTE; and atleast one adhesive layer for mechanically coupling said heat sinks tothe ground layer of said substrate, said adhesive layer comprising atleast one solder aperture, wherein aligning said at least one substratesolder aperture with said at least one adhesive layer solder apertureenables solder wetting in a first predetermined area between said firstheat sink and the ground layer of said substrate and in a secondpredetermined area between said second heat sink and the ground layer ofsaid substrate.
 29. The apparatus of claim 28 further comprising a firstdevice coupled to the topside of said substrate such that at least aportion of said first device overlaps said first heat sink and a seconddevice coupled to the topside of said second heat sink such that atleast a portion of said second device overlaps said second heat sink andwherein the CTE of said second heat sink is essentially matched to a CTEof said second device.
 30. The apparatus of claim 29, wherein said firstdevice is a power device, and said second device is a ceramic device.31. The apparatus of claim 30, wherein said first device is a radiofrequency transistor, and said second device is a load resistor.